As a method for manufacturing thin cast strips by means of continuous casting, there is provided a twin drum type continuous casting method described as follows. While molten metal is being supplied to a molten metal pool formed by a pair of cooling drums, which are rotated in the opposite direction to each other, and by a pair of side dams which are pressed against both end faces of these cooling drums, solidified shells are formed on the circumferential faces of the respective cooling drums which are being rotated, and the thus formed solidified shells are compressed to each other at the kissing point formed between the cooling drums so that the solidified shells are formed into a thin cast strip, which is wound into a coil on the downstream side.
Non-metallic inclusions such as slag are mixed in the molten metal supplied to the molten metal pool. Further, on a surface of the molten metal, metallic oxides are generated by oxidation. The thus generated non-metallic inclusions and metallic oxides are formed into a scum floating on the surface of the molten metal in the molten metal pool and caught from the meniscuses by the circumferential faces of the cooling drums and onto the surface of the cast strip. As a result, in the portion into which the scum has been-caught, cooling of the cast strip is delayed and, further, the cast strip can not be cooled uniformly. Accordingly, problems such as cracks, non-uniformity of structure and unevenness of acid pickling are caused. Further, unevenness of gloss is caused on products.
As a method of preventing the surface of molten metal in the molten metal pool from being oxidized, there is conventionally provided a method in which the molten metal pool is covered with a seal chamber, and non-oxidizing gas such as inert gas is supplied into the seal chamber so as to seal the molten metal face. This method is disclosed, for example, in Japanese Unexamined Patent Publication No. 3-198951. However, even when the molten metal face is sealed by this method, it is impossible to sufficiently prevent the generation and formation of scum.
On the other hand, as a method of preventing scum from being caught onto the circumferential faces of the drums and onto the surface of the cast strip, there is provided a conventional method in which a pair of scum dams extending in the width direction of the drums are arranged on both sides of a molten metal pouring nozzle being dipped in the molten metal so that the flow of scum to the cooling drum side can be prevented by the scum dams. This technique is disclosed, for example, in Japanese Unexamined Patent Publication No. 3-66450. However, even when the scum dams are provided as described above, it is impossible to prevent scum, which has been generated and formed in the periphery of the meniscus between the scum dams and the cooling drums, from being caught by the surfaces of the drums.
Especially, it is a problem that the scum is unevenly caught in the width direction of the cast strip. When a quantity of scum is large, there is a tendency that the scum is unevenly caught. When the casting time increases, the quantity of scum generated and formed is increased. Therefore, in the case where casting work is performed over a long period of time so as to enhance the productivity, the quantity of scum generated and formed is increased and the thus generated and formed scum tends to be unevenly caught.